1. Field of the Invention
This invention relates broadly to storage tanks. More particularly, this invention relates to portable storage tanks for temporary liquid storage.
2. State of the Art
Portable liquid storage tanks are used in an array of environmentally sensitive projects requiring temporary liquid storage. Such projects include contaminated ground water treatment, job site clean-up, hazard waste site clean-up, sludge pond clean-up and removal, oil and water separation, tank cleaning and maintenance, underground tank removal, repair and replacement, etc.
Various types of trailer transported or dolly and hitch transported portable storage tanks are utilized in these projects. The tanks are relatively large, capable of containing from 10,000 gallons to over 20,000 gallons of liquid. The liquids may be water, mud, sludge or other liquids with a heavy sediment, corrosive liquids, or other liquids which need to be transported away from or to a site. Due to the amount of liquid held within the tanks, the interior of the tanks will be subject to high pressure. Accordingly, the tanks must be relatively strong and are generally made from a strong rigid steel frame with steel plates affixed thereto to thereby provide a capable storage area.
Referring to prior art FIG. 1, one typical portable tank 10 includes an internal steel frame, generally 12, and a plurality of plates 13 attached to the outside of the frame. The frame 12 is formed by a relatively large number of upright side wall beam members 14 (a reduced number being shown for illustrative convenience), internal braces 16 which reinforce the side wall beam members 14, top braces 18 to support a roof portion 20, and lower bracing 24 across the lower portion of the tank which is provided with a floor 22 attached to the underside thereof. A conventional manway 26, typically approximately twenty-two inches in diameter, permits access into the interior of the tank.
The tank of prior art FIG. 1 has several disadvantages. First, the internal frame, particularly the internal braces, creates a dangerous barrier for workers who must physically enter the tank to clean the tank with water jets from high powered water hoses. Second, the junctions of the beam members, braces, and plates create interstitial spaces, which when the tank is filled with corrosive liquids, subject the surrounding locations of the tank to corrosion and may eventually breach the structural integrity of the tank. Third, upon removing a liquid from the tank, the interstitial locations and floor bracing make the tank particularly difficult to clean prior to receiving subsequent liquid contents. As a result, subsequent liquid contents of the storage tank can be contaminated by remnant liquids. In addition, the relatively small size of the manway creates difficulty for workers to enter and exit the tank. Moreover, sediment remaining in the tank after the liquid has been drained, e.g., remaining sludge, needs to be removed from the tank. However, the small size of the manways are inefficient for this sediment removal. Nevertheless, the small size of the conventional manway is dictated by the need to maintain structural stability of the plate through which the manway is provided when the tank is filled with liquid.
Other prior art tanks (not shown), such as the 10K Mobile Liquid Storage System available from Baker Tanks of Rancho Dominguez, Calif., or the FLUID BIN.TM. Roll-Off Box from V. E. Enterprises, Inc. of Springer, Okla., are similarly designed but utilize corrugated steel side walls to provide high strength to the side walls without necessitating internal bracing. This type of tank has severe drawbacks. First, such tanks are much more difficult to clean than tanks having flat wall sections. Water jets from high powered hoses do not satisfactorily clean the depths of the corrugations and the extra labor required for physical cleaning is extremely costly and time consuming. Second, the corrugated walls are susceptible to attack from corrosive liquids.
U.S. Pat. No. 5,582,311 to Bartenstein et al. discloses another tank design which attempts to eliminate several disadvantages found in prior tanks. Referring to prior art FIG. 2, the Bartenstein et al. tank 50 includes walls 52 and a frame 54 located outside the walls. The frame 54 includes beam members 56 connected by moment resisting joints 58. The floor 60 of the tank 50, free from internal bracing, is also sloped in a shallow V to encourage the flow of liquid toward a drain (not shown). The interior of the tank 50, free of structure (no bracing beams) and having a sloped floor, permits workers to freely move about the tank and makes it relatively easier to clean with jets of water than other storage tanks. However, the corners of the tank are still formed at approximately ninety degree angles and have a tendency to retain fluid and particularly sediment settling from the fluids even after cleaning. In addition, the tank is labor intensive and costly to construct, requiring a complex assemblage of beams and steel plate side walls. Moreover, the Bartenstein et al. patent does not address the difficulty workers have entering into and exiting from tanks through manways.